Teradata - Hashing Algorithm

A row is assigned to a particular AMP based on the primary index value. Teradata uses hashing algorithm to determine which AMP gets the row.

Following is a high level diagram on hashing algorithm.

Following are the steps to insert the data.

• The client submits a query.

• The parser receives the query and passes the PI value of the record to the hashing algorithm.

• The hashing algorithm hashes the primary index value and returns a 32 bit number, called Row Hash.

• The higher order bits of the row hash (first 16 bits) is used to identify the hash map entry. The hash map contains one AMP #. Hash map is an array of buckets which contains specific AMP #.

• BYNET sends the data to the identified AMP.

• AMP uses the 32 bit Row hash to locate the row within its disk.

• If there is any record with same row hash, then it increments the uniqueness ID which is a 32 bit number. For new row hash, uniqueness ID is assigned as 1 and incremented whenever a record with same row hash is inserted.

• The combination of Row hash and Uniqueness ID is called as Row ID.

• Row ID prefixes each record in the disk.

• Each table row in the AMP is logically sorted by their Row IDs.

How Tables are Stored

Tables are sorted by their Row ID (Row hash + uniqueness id) and then stored within the AMPs. Row ID is stored with each data row.